1. Field of the Invention
This invention relates generally to digital to analog converters and more particularly to companded digital to analog converters having programmable Mu- or A-law capability.
2. Description of the Prior Art
Companding (compression/expansion) digital to analog converters are commonly used in pulse code modulation (PCM) to encode voice signals into an eight bit binary code utilizing either the Mu-255 or A-law conversion. Previously, such companding digital to analog converters have utilized two binary-weighted capacitor arrays wherein one array was used for step bit encoding and the other array was used for chord bit encoding as discussed by Paul R. Gray, David A. Hodges, John P. Tsividis and Jacob Chacko, Jr. in an article entitled "Companded Pulse-Code Modulation Voice Coder Using Monolithic Weighted Capacitor Arrays", in the IEEE Journal of Solid State Circuits, December, 1975, pp. 497-499. A unity gain buffer amplifier connected the two capacitor arrays, and three transmission gates were required for switching each capacitor in the chord array to one of three bus lines. The physical arrangement of the switching structure required tunneling in order to fabricate an integrated circuit. Furthermore, the associated logic circuitry required the use of a sequencer and was complex.
Several variations of companding digital to analog converters have been used. Single capacitor arrays were utilized for both the step and chord determinations. Converter circuits with a single array of capacitors have the distinct disadvantage of having capacitor ratios of 128:1 which are the source of error since precise large capacitor ratios are difficult to obtain. A circuit using a switched resistor ladder for the step bit determination and a capacitor array for the chord bit determination was discussed by J. T. Caves, C. H. Chan, S. O. Rosenbaum, L. P. Sellars and J. B. Terry in an article entitled "A PCM Voice Coder with On-Chip Filters", in the IEEE Journal of Solid State Circuits, February 1979, pp. 65-67. By using the resistor array to convert the step bits, the unity gain buffer amplifier was eliminated and only a single reference voltage was required, but this and all other comparable circuits continue to have all the previously mentioned disadvantages.